1. Field of the Invention
The present invention relates to a power transmission device, a power reception device, a power supply system, and a power transmission method.
2. Description of the Related Art
In recent years, short range radio communications by NFC (Near Field Communications) are in widespread use. Using the short range radio communications, power supply systems that supply power (transmit power) in a non-contact manner are studied. Such a power supply system is constituted of a power transmission device and a power reception device, which include information transmission means and non-contact power transmission means to perform information transmission and power transmission by the short range radio communications.
To maintain communication quality, the information transmission means and the power transmission means operate in an exclusive manner. However, when the exclusive control takes the form of disabling starting of operation of one of the means as long as the other operates, even if a battery is exhausted during information transmission, charging of the battery is not started immediately, thus interfering with stable operation of the communication device. Therefore, there is proposed a communication device in which information transmission is performed in a time sharing manner, and a sleep period, except for an active period for performing a radio communication process, is assigned as a power supply period, in order to automatically perform the exclusive control between the radio communication process and the charging process (for example, Patent Document 1: Japanese Patent Application Laid-Open No. 2009-253649).
The power reception device can exchange general information with PCs, as well as exchange information for power supply control with the power transmission device. For example, when a watch that automatically stores positional information as a terminal device (or a power reception device) and a device connected to a PC functions as a battery charger (or a power transmission device), there is conceivable an instance where the PC captures the positional information from the watch, while the battery charger charges up the watch.
In such an instance, to capture the positional information from the watch into the PC after a while from the start of charging, the general information is exchanged after the battery charging is forcefully suspended. After that, charging control information including certification and the like is exchanged again, and thereafter the battery charging is restarted. This causes a reduction in power transmission efficiency and an increase in substantial charging time. Since the general information cannot be exchanged during the exchange of the charging control information, initialization is required to be performed after waiting for a certain time period, thus causing an increase in a reaction time for information exchange.
To solve the problem described above, the present invention has an object to provide a power supply system that can exchange information without a reduction in power transmission efficiency.